Well, as someone who used to work for ATA (I left on my own 2 years ago), perhaps I can help. I interviewed with ATA a few weeks ago for a position here in Indy (which even if they offered it to me the hours wouldn't have worked out) and I asked about the fleet plans. Here is what I was told. . .
We don't know. Specifically, the plans change from week to week. The latest rumor is that ALL of the 757's (200's and 300's) will be sent back. They want to get some 767's to replace the L-1011's, but they are too expensive right now. The L-1011's have outlived their useful life and spare parts are incredibly difficult to find, but because of the cost of a 767, they might put a few through D checks and keep them flying for the military. They want some 737-300's but finding a way to pay for them is proving difficult.
I was also told that while the codeshare with WN is doing well in terms of number of customers, ATA isn't even close on the customer side of things, i.e., on-time flights, customer complaints, etc.. Finally, after my interview I went and talked to a friend who still works here in Indy and she's been told that the company was supposed to break even in March but won't come even close. Also, ATA has enough cash to get through September and after that she's been told that's it - which is why she's trying to find another job. So if want to take another ATA flight, do it quick.
"If you are looking for perfect safety, you will do well to sit on a fence and watch the birds. But if you really wish to learn you must mount a machine and become acquainted with its tricks by actual trial." - Wilbur Wright, 1901